There is well known a hybrid working machine e.g. a power shovel using a parallel driving system (see Japanese Unexamined Patent Publication No. 10-42587)
In the parallel driving system, a hydraulic pump, and a power machine functioning as a generator and an electric motor are connected in parallel to an engine serving as a common power source. The hydraulic pump drives a hydraulic actuator, while the power machine functions as a generator to store power in a power storage device. Also, the power discharged from the power storage device, when needed, allows the power machine to function as a motor for assistance of the engine.
As the power machine, there may be used a single machine which functions both as a generator and an electric motor, or individual machines, one of which is a generator and the other is an electric motor.
The hybrid working machines enable to reduce a load of the engine and to drive the engine in a high performance range so as to realize energy-saving.